Before Italy had national laws, local people served as Carabinieri who enforced territorial laws.
Today the Carabinieri serve as a special branch of the army and deal with criminal investigations in serious crimes and are considered military police.
When Italy did institute their National Police (Polizia di Stato), instead of replacing the Carabinieri they learned to work in conjunction with them.
For the most part, it is more common to see the National Police monitoring major roads and towns while the Carabinieri are dispersed throughout a territory.
The carabinieri or gendarmerie, some 26,500 in number, are part of the standing army; they are recruited from selected volunteers from the army.